1. Field of the Invention
The present invention relates to a panel inspection apparatus and inspection method. Specifically, the present invention relates to a panel inspection apparatus and inspection method for measuring resonant frequencies of a panel formed as a press-formed product and determining whether the quality of the panel is good based on the resonant frequencies.
2. Background Art
When a panel is formed by press-forming a plate material, the panel may be constricted or cracked. Such constrictions and cracks easily occur in particular at portions such as flange and corner portions at which distortion is concentrated during press-forming. However, at some portions, the constriction and crack occur unexpectedly, so that it is difficult to estimate the occurring locations. Therefore, the inspection of constrictions and cracks must be performed extensively for the entire formed item, and may take time.
Conventionally, an inspection apparatus which can perform an inspection of constrictions and cracks of a panel in a short time has been proposed. As such an inspection apparatus, for example, in JP-A-09-171008 and JP-A-2007-147512, inspection apparatuses which vibrate a panel, detect the vibrations of the panel, and based on a frequency characteristics unique to the panel extracted from the detected vibrations, determine whether the quality of the panel is good, are shown.
FIG. 24 is a histogram showing distributions of plate thicknesses of a plurality of plate materials included in one standard.
In FIG. 24, the horizontal axis indicates the plate thickness of the plate material, and the vertical axis indicates frequency. As shown in this figure, plate materials to be used as materials of press-formed products have variation in plate thickness although they are included in the same standard.
However, if the plate materials to be used as materials have such variation in plate thickness, it cannot be distinguished whether a change in frequency characteristics of a panel is caused by the presence of a constriction and a crack or caused by variation in plate thickness of plate materials. In the above-described Patent documents 1 and 2, such variation in plate thickness is not considered, and therefore, determination on the quality of the panel may be different from actuality.